Circuit for indicating variations in mark-to-space ratio

ABSTRACT

A phase-comparing circuit has two paths switched on and off alternately by the signals to be compared and a meter measures the average difference between the currents in the paths. Both currents are partly determined by a common transistor and by a respective transistor in the two paths, and temperature changes affect the three transistors equally to give temperature compensation to the meter reading.

United States Patent [72] Inventors Stuart Alexander Andrews MorganWynne Lewis, Bushey Heath, both Elstree;

of England Appl. No. 41,721 Filed May 27, 1970 Patented Dec. 7, 1971Assignee Rolls-Royce Llmlted Derby, England Priority Nov. 9, 1966 GreatBrltaln Contlnuatlon ol appllcatlon Ser. No. 681,875, Nov. ll, 1967, nowabandoned. This application May 27, 1970, Ser. No.

CIRCUIT FOR INDICATING VARIATIONS'IN MARK-TO-SPACE RATIO 3 Claims, 1Drawing Fig.

US. Cl

Lacy; Elec. Engn, pp. I48- l5l); Mar. I967.

Primary Examiner- Alfred E. Smith Att0rney Mawhinney and MawhinneyABSTRACT: A phase-comparing circuit has two paths switched on and offalternately by the signals to be compared and a meter measures theaverage difference between the currents in the paths. Both currents arepartly determined by a common transistor and by a respective transistorin the two paths, and temperature changes affect the three transistorsequally to give temperature'compensation to the meter readmg.

22 Ave CIRCUIT FOR INDICATING VARIATIONS IN MARK-T- SPACE RATIO This isa continuation of application Ser. No. 68l,875,filed Nov. 1 l,l967,andnow abandoned.

This invention relates to a circuit for comparing the phase of twosignals, for example the two square wave signals which can be derivedfrom the two relatively angularly movable discs employed in some formsof torque meter. Thus the invention is applicable to the torque meterthe subject of British Pat. Nos. 968,503 and 43,l 19/66 but this is onlyone application of the invention.

According to the present invention the circuit includes two conductingpaths arranged to conduct alternately in according with the signals tobe compared each including an electronic device for partiallydetermining the current when that path is conducting, and means formeasuring the mean difference between the currents in the two paths, thecurrent value in the paths when they conduct being also partiallydetermined by a third electronic device, and the three electronicdevices exhibiting similar changes in their characteristics withtemperature whereby a temperature change of the first and secondelectronic devices which would cause a change in current value in thepaths compensated for by the effect of the equivalent change intemperature of the third device. Each path may also include a transistoror other switch device for controlling conduction of that path independence on the corresponding signal.

Prior phase comparing circuits have tended to be unreliable when used ata temperature other than a predetermined temperature partly due to thechange in the characteristics of the valves, transistors, or equivalentelectronic devices, used in them, but by using two suchdevices todetermine the current value and arranging for both devices to change inthe same way with temperature it can be arranged for the current valueto remain more nearly the same at different temperatures.

Preferably the electronic devices are three-electrode transistors, andin one embodiment of the invention employing transistors, the bases orcontrol electrodes are biassed by a common reference voltage and the twoconducting paths have respective resistive portions connected betweenanother electrode of the third device and corresponding electrodes ofthe first and second devices. If the voltages of the electrodes of thethree devices in the current paths all change together with temperatureit follows that the potentials drops across the resistive portions andhence the currents in them will remain substantially constant in spiteof temperature changes.

One of the resistive portions may be adjustable to set the zero so thatthe currents in the two conducting paths are equal and the meandifference between them is zero when the signals have no phasedisplacement from a nominal phase relationship.

Similarly the bias on the control electrode in the third device may beadjusted to set a meter of the mean current difference to have fullscale deflection at a desired phase difference between the two signals.

Moreover the bias on the control electrodes of the devices can bederived from a circuit including a resistor with a hightemperaturecoefficient of resistance, which is subjected to the same temperature asthe instrument providing the two signals for comparison, so that therecan be some built-in compensation for temperature changes in theinstrument itself.

Preferably a meter is connected to carry the currents in the two pathsin opposite senses so that the indication will be of the mean differencebetween the currents in the two paths.

The invention may be carried into practice in various ways and oneembodiment will now be described by way of example with reference to theaccompanying drawing of which the single FIG. is diagram of arnark-to-space ratio determining circuit. In the application beingdescribed, a torque measured by a torque meter determines themark-to-space ratio of a square wave, and the circuit to be describedprovides a current dependent on this ratio.

Two signals are supplied to the bases of respective switchingtransistors I and 2. The signals are in the form of rectangular waveshaving complementary marks and spaces, that is to say, each wave hasmarks coincident with spaces in the other wave and appear at a service2! having two terminals. For zero torque, the mark-to-space ratios areunity but as the torque is increased, the ratios depart increasinglyfrom unity.

The transistors l and 2 are connected with two other transistors 3 and 4in two parallel paths, and DC meter movement 5 is connected across thejunctions between the two series transistors in each path. The emittercircuit of transistor 3 includes a resistive load comprising a resistorR6 and an adjustable resistor R7 along with a resistor R8. Resistor R8is common to the emitter circuit of transistor 4, which also includes aresistive load comprising a resistor R9. The transistors I and 2 arebeamed to be switched on by the positive marks of the waves they receiveand to switch off during the spaces. Because the waves are complementarythis will mean that transistor 1 will be on when transistor 2 is off andvice versa. Transistors 3 and 4 are biased at 11 to be on continually.

The arrangement is such that when transistor I is switched on by theleading edge of the mark applied to it, the current through thetransistor 1, will flow in two paths, one containing the transistor andresistor 3 and resistors R6, R7 and R8, and the other containing themeter 5, the transistor 4, and the resistors R9 and R8. A source ofcurrent 22 is connected between the end of the resistor R8 and thecommon collectors of the transistors l and 2.'When the space of the waveat the base of transistor I occurs, this transistor switches off. At thesame time the base of the transistor 2 experiences the leading edge of amark and in switched on causing current flow through the transistor 2and then through one path containing the transistor 4 and resistors R9and R8 and another containing the meter 5, the transistor 3, and theresistors R6, R7 and R8.

Thus. the meter 5 conducts currents in opposite directions in the twocases.

If the mark-to-space ratio is exactly unity the meter will conductcurrent in opposite directions for equal times and'if the resistor R7 isadjusted to make the currents through transistors 3 and 4 the same, azero indication of the meter, which represents the time average currentin it, will indicate this ratio of unity or zero torque. If now themark'to-space then in each square wave cycle the meter will conductcurrent in direction for a certain time and in the other direction for adifferent time, and there will be a net average current in the meter toproduce an indication of the degree of torque change.

The bases of the transistors 3 and 4 are biassed from a precisionvoltage reference 11 and the same reference is used to bias the base ofathird or compensating transistor 12 by use of potential dividerconsisting of a fixed resistor R13, an adjustable resistor R14 forsetting the full scale deflection of the meter and a temperaturecompensating resistor R15. The emitter of transistor 12 is connectedbetween resistor R8 on the one hand and resistors R7 and R9 on theother. The emitter voltages of the transistors 3, 4 and 12, are set bytheir base biases and the difference between the emitter voltages of thetransistors 3 and 4 and that of the transistor 12 determines the valueof the current flowing in the two conducting paths when conduction takesplace. By setting the resistor R14 this current value can be set to givefull-scale deflection of the meter 5 for a desired mark-to-space ratioequivalent to be desired torque.

If now, the temperature changes, so that the transistors 3, 4 and 12operate at a different part of their characteristics, this will notupset the accuracy of the instrument provided the three transistors arechosen to have similar temperature characteristics relating theiremitter voltages to their base biases Thus, if due to such a temperaturechange the voltages of the emitters change the emitter voltages of allthe transistors will change. Therefore the potential difference acrossthe resistor R6 and R7 on the one hand and R9 on the other hand willremain the same and the current through transistor 3 and 4 will beunaffected by the temperature change so that the meter reading will bealso unaffected.

The resistor R is arranged to be subjected to the same temperature asthe torque shaft which with a generator provides indirectly the signalson the bases of the transistors l and 2, and it is chosen to have ahigh-temperature coefficient of resistance, so that compensation isobtained for changes in the mark-to-space ratio of the signals due totemperature changes of the torque shaft from which the signals arederived.

What is claimed is:

l. A circuit for indicating variations in the mark-to-space ratio of asquare wave signal comprising:

l. a source of a square wave signal of variable mark-tospace ratio and asource of current,

2. first and second switches, first and second connection points, andfirst and second main transistors,

3. connections from said source through:

i. in series connection, the first switch, the first connection point,the first main transistor,

ii. in series connection, the second switch, the second connectionpoint, the second main transistor,

4. a meter connected between the first and second connection points,said first and second switches respectively being arranged to beswitched on by a first portion of said square wave signal and by thecomplementary portion of said signal, whereby when said first portion ispresent current flows:

a. through said first switch, said first connection point and first maintransistor, and,

b. through said first switch, said first connection point, said meter,said second connection point and said second main transistor; and whensaid complementary portion is present current flows;

c. through said second switch, said second connection point and saidsecond main transistor, and,

d. through said second switch, said second connection point, said meter,said first connection point and said first main transistor;

so that the meter indicates variations in said mark-to-space ratio.

2. A circuit according to claim 1 including a compensating transistorhaving similar temperature characteristics and being arranged in thesame environment as the first and second main transistors and being conrected to compensate for changes in current in the main transistors dueto changes in temperature.

3. A circuit according to claim 2 including a first resistive load and athird connection point in series connection with the first maintransistor; a second resistive load and said third connection point inseries connection with said second main transistor; and a thirdresistive load connected to said third connection point; biasing meansarranged to apply a related bias to the bases of both said main and saidcompensating transistors; for establishing that changes in the base toemitter voltages of both said main and said compensating transistors dueto ambient temperature changes cause no change in potential drop acrossthe first and second resistive loads and do not alter the current ineither of said main transistors.

1. A circuit for indicating variations in the mark-to-space ratio of asquare wave signal comprising:
 1. a source of a square wave signal ofvariable mark-to-space ratio and a source of current,
 2. first andsecond switches, first and second connection points, and first andsecond main transistors,
 3. connections from said source through: i. inseries connection, the first switch, the first connection point, thefirst main transistor, ii. in series connection, the second switch, thesecond connection point, the second main transistor,
 4. a meterconnected between the first and second connection points, said first andsecond switches respectively being arranged to be switched on by a firstportion of said square wave signal and by the complementary portion ofsaid signal, whereby when said first portion is present current flows:a. through said first switch, said first connection point and first maintransistor, and, b. through said first switch, said first connectionpoint, said meter, said second connection point and said second maintransistor; and when said complementary portion is present currentflows; c. through said second switch, said second connection point andsaid second main transistor, and, d. through said second switch, saidsecond connection point, said meter, said first connection point andsaid first main transistor; so that the meter indicates variations insaid mark-to-space ratio.
 2. first and second switches, first and secondconnection points, and first and second main transistors,
 2. A circuitaccording to claim 1 including a compensating transistor having similartemperature characteristics and being arranged in the same environmentas the first and second main transistors and being connected tocompensate for changes in current in the main transistors due to changesin temperature.
 3. A circuit according to claim 2 including a firstresistive load and a third connection point in series connection withthe first main transistor; a second resistive load and saId thirdconnection point in series connection with said second main transistor;and a third resistive load connected to said third connection point;biasing means arranged to apply a related bias to the bases of both saidmain and said compensating transistors; for establishing that changes inthe base to emitter voltages of both said main and said compensatingtransistors due to ambient temperature changes cause no change inpotential drop across the first and second resistive loads and do notalter the current in either of said main transistors.
 3. connectionsfrom said source through: i. in series connection, the first switch, thefirst connection point, the first main transistor, ii. in seriesconnection, the second switch, the second connection point, the secondmain transistor,
 4. a meter connected between the first and secondconnection points, said first and second switches respectively beingarranged to be switched on by a first portion of said square wave signaland by the complementary portion of said signal, whereby when said firstportion is present current flows: a. through said first switch, saidfirst connection point and first main transistor, and, b. through saidfirst switch, said first connection point, said meter, said secondconnection point and said second main transistor; and when saidcomplementary portion is present current flows; c. through said secondswitch, said second connection point and said second main transistor,and, d. through said second switch, said second connection point, saidmeter, said first connection point and said first main transistor; sothat the meter indicates variations in said mark-to-space ratio.